This invention relates to power transistor switching circuitry and, more particularly, to such circuitry which is used in the output stage of apparatus such as power inverters.
Drive circuits for power transistors operating in the switching mode must supply ample drive current to assure low voltage drop during the ON state and must provide means for causing fast turn-off of the power transistor. A known way of controllably and reliably applying base drive current to transistors operating in a high current, switching mode is by the use of a current controlled feedback transformer drive circuit. However, with such a circuit, the relatively high drive current requirements and the necessary fast turn-off circuitry may result in bulky circuit configurations in which stray inductance can cause severe voltage spikes during switching.
Electronic inverters commonly include a plurality of output transistor switches connected in a bridge arrangement. Output power poles are formed between a pair of power switching transistors which are electrically connected in series between a pair of DC conductors. When switching the transistors connected to a single inverter output power pole, it is important to use caution to avoid turning on a transistor while its counterpart is capable of conduction. This is sometimes accomplished by providing an underlap in the control signals. While this works fairly well, it can produce periods where the pole is not conductive and therefore at an undetermined voltage. Thus, it is desirable to provide an interlock to inhibit turn-on of either transistor until the other is positively off.